Potential Conflict between ThreadingInstrumentor and Gevent Workers in Flask/Gunicorn Setup

Describe your environment

OS: Ubuntu, centos(e.g, Ubuntu)
Python version: (e.g., Python 3.10)
Package version: (e.g., 0.46.0)
OpenTelemetry Instrumentation: opentelemetry-instrumentation-threading
WSGI Server: Gunicorn (with gevent worker class)
Application: Flask

What happened?

Enabling ThreadingInstrumentor appears to conflict with gevent workers when running a Flask app under Gunicorn.
Code Snippet (manager_test.py)

import logging
import sys
import threading
from flask import Flask
from opentelemetry.instrumentation.threading import ThreadingInstrumentor

ThreadingInstrumentor().instrument()  # Enables instrumentation

app = Flask(__name__)
logging.basicConfig(
    level=logging.INFO,
    format='%(asctime)s %(levelname)s %(process)d %(thread)d %(name)s %(filename)s:%(lineno)d:%(funcName)s - %(message)s',
    stream=sys.stdout
)

def test_timer(pos):
    logging.info("-------->>>> Timer task executed, pos=%s", pos)
    # Periodically reschedule itself
    threading.Timer(1, test_timer, [pos]).start()

# Start initial timer
threading.Timer(1, test_timer, [pos]).start()

@app.route('/vi/health', methods=['POST', 'GET'])
def healthcheck():
    return 'ok'

if __name__ == '__main__':
    app.run('0.0.0.0', 8080)

Gunicorn Configuration (manager.conf.py)

daemon = False
chdir = '/opt/app/'
bind = '0.0.0.0:8080'
reuse_port = True
proc_name = 'mms_manager'
workers = 1
worker_class = 'gevent'  # Problem occurs with this
timeout = 60
graceful_timeout = 300

Steps to Reproduce

Run app with:
gunicorn -c manager.conf.py manager_test:app

Expected Result

The test_timer() function should still execute periodically when using worker_class = 'gevent'.

Actual Result

The scheduled test_timer() function ​never executes​ when using worker_class = 'gevent'.

Additional context

Switching to worker_class = 'gthread' resolves the issue immediately:

• Timer tasks execute as expected
• All other functionality remains unchanged

Would you like to implement a fix?

None

Start with manager_test.py and manager.conf.py, then reproduce with gunicorn -c manager.conf.py manager_test:app using the gevent worker class. Compare the behavior with gthread and inspect how ThreadingInstrumentor interacts with the scheduled threading.Timer. Done means test_timer() executes periodically under gevent without changing the existing Flask endpoint behavior.